In a space mission as complex as this one -- the high-stakes repair of the Hubble Space Telescope -- the Endeavour crew had to learn not only what makes a billion-dollar space observatory tick but how to turn back their own internal clocks. They did it with something called "light therapy."

The Endeavour crew's sleeping patterns had to change in order for the mission to reach the telescope, which is orbiting the Earth at 17,000 miles an hour. The shuttle was expected to catch up with the telescope about 3:45 a.m. today after a two-day orbital chase following Thursday's launch. The crew today is to begin efforts to haul Hubble into the shuttle bay for several days of repairs.

"We use bright lights . . . to shift the crew. We can shift them to any time zone we want in about three days," said Dr. Richard G. Jennings, a NASA flight surgeon familiar with the therapy program.

In the week before they blasted off into space, Endeavour's seven astronauts found themselves in a windowless building at the Johnson Space Center, a special facility in which shuttle crews are quarantined before a mission. There, for several hours a day, the crew was exposed to a special dose of light configured by specialists in circadian medicine. The idea was to shift their body clocks back about 12 hours, as though they were living in Tokyo.

"But the difference is, you're not in Tokyo, and you don't have Tokyo sunlight," said Dr. Jennings. "You have Houston light . . . so we're using bright lights to get [them] adjusted."

Overhead ceiling lights, similar to fluorescent lights in an office, shower crew members with "about one-tenth of the light you would be exposed to on a clear day in Baltimore," said Dr. Charles Czler, a circadian medicine specialist in Boston.

"We're tricking the clock in the human brain. . . . Once we shift them to the appropriate time, they will stay there," said Dr. Czler.

Previous shuttle crews have had to work at night and sleep in the day, Dr. Jennings said. Some crews have worked split shifts, during which half the crew began work at noon, the other at midnight, he said.

NASA began using light therapy in 1990. Before then, he said, "The crews just had to tough it out."

NASA convened a panel of circadian specialists who recommended the light therapy technique in the week before a flight, Dr. Jennings said.

The recommended dose? 10,000 lux of light (A lux is a measure of the brightness of light equal to that of a candle held one foot away.)

The Endeavour crew's eight-hour sleep cycle had to begin at 11 a.m. Over the course of the week, the plan was to move each astronaut's clock westward, toward Hawaii, said Dr. Jennings. At the start of the quarantine week, the crew received about five hours of light therapy a day, beginning about 1 a.m. By week's end, it had been reduced to about two hours.

"Once we get them moved, the light therapy isn't quite as long," said Dr. Jennings. "There are other times [of the cycle] that it's critical that you don't get any light at all. We use goggles on occasion that will block out 99.9 percent of transmitted light."

Because the crew works during the quarantine period, the special light panels are located throughout their crew quarters in Houston so that astronauts can receive the therapy while working at a computer station or exercising, said Dr. Jennings.

Although Dr. Jennings said the therapy has helped shuttle crews in the past, NASA has never scientifically documented whether their new circadian rhythms actually stay synchronized while in orbit. But while in quarantine, Dr. Jennings said, the crews who have undergone the light therapy have experienced less sleep deprivation.

During the Endeavour mission, the crew sleeps in hammock-like sleeping bags in an area of the shuttle that has only two 4-inch-wide windows. Due to the swift change in day and night in space, the shuttle windows can be covered to block out sun- or moonlight.

Still, said Dr. Czler, the Harvard specialist, "It really is tough to sleep in space."

He cited a NASA survey of 58 crew members from nine shuttle missions. Most of the crew members suffered from sleep disruption, he said, sleeping only six hours, instead of the planned 7.9 hours.

Plus, he said, 50 percent of those surveyed who worked dual shifts and 19 percent on single shifts used sleeping pills during their missions.

For the astronauts who are too excited to sleep on this mission, sleeping pills are available aboard Endeavour.

FOR UPDATES

To hear updates on the space shuttle Endeavour's mission to repair the Hubble telescope, call Sundial, the Baltimore Sun's telephone information service, at (410) 783-1800 (268-7736 in Anne Arundel County, 836-5028 in Harford County, 848-0338 in Carroll County). Punch in the four-digit code 6116 after you hear the greeting.